There are many shades of gray between pain and sprain.
If you just completed 20 miles, I'm not too worried about the latter. Sprains tend to keep your attention for a while (watch for swelling).
You've already gone farther than most people will ever attempt in their entire lives, so it can't be too bad. Almost all of us have experienced various pains as we "break in" our running form. If the pain goes away after rest (in some cases, ice), and is only early in (and after) your longest runs, I wouldn't worry too much at this point in time. One of the purposes of your long runs is to gradually build up your stamina so you can preserve your form longer and reduce the chance of injuries and pain in your target event and/or longest runs. Monitor this!
Meanwhile, it's a good idea to check a few things before you go further:
Pain on the inside of the ankle can be a sign of over-pronation, because the tendons for your supinating (anti-pronation) muscles wrap around the inside of the ankle on the way to the bottom of the foot. Too much wear and tear here should make you take a look at your footstrike. Are the toes pointed straight forward when your foot hits the pavement? Is there excessive wear on the inside edge of the sole of your running (or other) shoes?
It would be normal for your form to get a little sloppier as you push the edge of the envelope in your longest runs, so you want to check these things earlier when your legs are fresh.
Finally, are you running mostly on the left side of a road that is sloped to the left for drainage? This would load the supinating muscles of your right leg more as you keep yourself level. The left side is usually safest from a traffic perspective, but a neutral surface is much better for your legs. Find a park or other protected area for these long runs. Your Marathon is usually protected to allow safe running on a neutral surface, but you are on your own the rest of the time. You must choose your running surface wisely. I do my shorts and mediums on the road, mixing the sides to match traffic flow (even grabbing the center when I can), but I do my speedwork and long runs in parks whenever possible to find safe, neutral ground.
No matter where you run, always look behind before moving left or right - especially if you are wearing headphones. Bicyclists can nail you too, since they are often competing for the same surface and rarely announce their presence.
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